1. Field of the Invention
The invention generally relates to the manipulation of droplets. In particular, the invention relates to the manipulation or actuation of droplets in a microfluidic circuit.
2. Description of the Related Art
The handling of liquids is fundamental to many fields in science, such as biology and chemistry. To efficiently perform relatively large numbers of tests, miniature laboratory environments have been produced. Conventional miniature laboratories can include relatively intricate, complex, and expensive micro-machined pumps and valves.
One known technique which appears to eliminate the need for micro-machined pumps and valves actuates droplets by electro-wetting. See Cho, S., et al, “Towards Digital Microfluidic Circuits: Creating, Transporting, Cutting and Merging Liquid Droplets by Electrowetting-Based Actuation,” Technical Digest, 15th IEEE International Conference on Micro Electro Mechanical Systems (2002), pp. 32-35. Disadvantageously, conventional electro-wetting techniques require relatively large and expensive control electrode arrays. In addition, it may be desirable to utilize alternating current (AC) biasing, which can be relatively difficult to switch in an array. As a result, conventional electro-wetting techniques are relatively expensive and can be cost-prohibitive in applications where large amounts of tests are conducted, and the miniature laboratory environments are discarded after a single use. Moreover, the simultaneous control of multiple droplets using conventional electro-wetting techniques may not be achievable as a practical matter due to the impracticality of independently controlling a large number of AC biased electrodes.
One known technique coats a surface with a chemical that reacts with light to change a contact angle and thereby manipulate a droplet. Disadvantageously, such chemical reaction techniques can take on the order of minutes to react and are too slow for practical use. See Ichimura, K., et al., “Light-Driven Motion of Liquids on a Photoresponsive Surface,” Science, Vol. 288 (June 2000), pp. 1624-1626. In addition, at least one such technique cannot manipulate water droplets. See Ichimura, K., et al.
Another known technique with zinc oxide (ZnO) or with titanium oxide (TiO2) can change the contact angle of an aqueous or water-based solution. See Sun, et al., “Photoinduced Surface Wettability Conversion of ZnO and TiO2 Thin Films, J. Phys. Chem. B, Vol. 105 (February 2001), pp. 1984-1990. Disadvantageously, ultraviolet light is used to generate the chemical reaction. See Sun, et al. Ultraviolet light can be harmful to biological organisms. Further disadvantageously, the chemical reaction is slow and may require at least 20 minutes to occur. See Sun, et al. Thus, while such chemical reaction techniques may be applicable to “anti-fogging glass and self-cleaning construction materials,” see Sun, et al., such chemical techniques may be ill suited to laboratory environments.
What is needed is an efficient and low-cost technique to build an electro-wetting surface.